A light-emitting element including an EL layer between a pair of electrodes (also referred to as an organic EL element) has characteristics such as thinness, light weight, high-speed response to input signals, and low power consumption; thus, a display including such a light-emitting element has attracted attention as a next-generation flat panel display.
In a light-emitting element, voltage application between a pair of electrodes causes, in an EL layer, recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance (organic compound) contained in the EL layer into an excited state. Light is emitted when the light-emitting substance returns to the ground state from the excited state. The excited state can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from a singlet excited state is referred to as fluorescence, and light emission from a triplet excited state is referred to as phosphorescence. The statistical generation ratio of S* to T* in the light-emitting element is considered to be 1:3. Since the spectrum of light emitted from a light-emitting substance depends on the light-emitting substance, the use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various colors.
In order to improve element characteristics of such a light-emitting element, improvement of an element structure, development of a material, and the like have been actively carried out (see Patent Document 1, for example).